Exploration for hydrocarbons is a risky business. There is no guarantee that having identified an area likely to contain hydrocarbons, commonly referred to as a prospect, hydrocarbons will be extracted. Hydrocarbons and in particular oil and natural gas accumulate and form reservoirs in sedimentary basins in the Earth's crust. The oil and gas will desire to permeate through the sedimentary basin as a result of density and pore pressure differences and from compressional stresses generated within the Earth's crust. The oil and gas will tend to rise through the sedimentary basin until stopped by a seal, such as a layer of shale, where it will accumulate and form a reservoir.
The process of successfully extracting hyrdocarbons requires an appreciation of the stresses acting across the prospect. The mutually perpendicular compressional components of the stresses acting across any prospect may be expressed as Sv (vertical stress), SH (maximum horizontal stress), Sh (minimal horizontal stress). Whilst an appreciation of these stresses is required for various stages of the exploration and extraction process, appreciation is particularly important when drilling an extraction wellbore. More specifically when these stress components are not equated they tend to deform the wellbore cross section from a circle to an ellipse, a phenomenon known as wellbore breakout, which in some cases can lead to the collapse of the wellbore.
As discovery rates continue to decline, emphasis is turning from new basins and plays to smaller intra-basin discoveries requiring more detailed understanding of basin forming faults and their local stress effects on traps and trap geometries. Improved oil recovery is not only about finding new fields, but also demands detailed stress information for horizontal wellbore stability in order to economically and effectively increase reserves and recovery rates by extracting new oil from old fields. As a result, expensive wellbore based measurements have been deployed over the past 15 years. These precision measurements have then been averaged between wellbores for stress prediction but stress directions are known to vary abruptly by up to 90° over distances of less than 2 kilometers.
International patent application PCT/AU01/00568 published as WO01/90783 assigned to Petrecon Australia Pty Ltd and herein incorporated in its entirety by reference disclosed an improved solution involving the seismic recognition of globally synchronous compressional pulses. Seismic reflection data can be interpreted to suggest that the last period of compression producing these structures commenced in the Pliocene geological epoch some five million years ago, and that similar compressional pulse periods have occurred repeatedly since at least the Early Triassic some 240 million years ago. When comparing the seismic data from locations around the Earth the pulse periods can be interpreted as globally synchronous. FIG. 1 is a table showing the periods and locations of globally synchronous compressional pulses. This recognition provides a workflow for stress consistent seismic interpretation which can predict horizontal and vertical changes in the direction of the maximum horizontal compressional component of a stress SH (SHD).
It is an object of this invention to provide a method of proving a quantitative estimation or prediction of the magnitude of the stress components acting within a prospect pre-drill.
Other objects and advantages of the invention will become apparent to those of ordinary skill in the art having reference to the following specification together with its drawings.